Gas hydrate is a solid ice-like substance composed of water and gas in which gas molecules are encaged in the rigid lattice of water molecules. Methane hydrate (CH46H2O) consists of about 85% water on a molecular basis. In nature, gas-hydrate-bearing formations are generally multiphase systems consisting of a formation grain matrix with water, gas and hydrate occupying the pore space. Methane hydrate occurs commonly in marine and permafrost sedimentary environments with relatively low temperatures and high formation pressures. Because of its efficient molecular-scale packing, a unit volume of methane hydrate can store more than 160 volumes of free gas equivalent at standard temperature pressure (STP). In addition to representing a potentially vast energy resource for the future, the possible release of large amount of methane gas from geological reservoirs may be an important factor affecting global climate change and local terrain/slope stability.
Although its large natural occurrence has been qualitatively investigated, methods of accurate quantitative assessment of in-situ gas hydrate amounts still need to be improved. Accurate estimation of in-situ hydrate saturation is needed at least for thermodynamic modeling of hydrate dynamics to understand its initiation and dissociation in natural environments.
An evaluation of the properties of gas hydrate-bearing formations has been previously reported for results obtained from downhole log data from a gas hydrate research well. One particular well that has been used to collect data is the JAPEX/JNOC/GSC et al. Mallik 5L-38 Well located at (69°27′39.30″N, 134°39′38.90″W) in the Mackenzie Delta, Northwest Territories, Canada. The base of permafrost in the immediate vicinity of the well is estimated to be at a depth of 600 m beneath ground level. Gas hydrate observed in Mallik 5L-38 Well occurs around the depth of about 1000 m below the ground surface, within the Oligocene, Miocene, and Pliocene sediments composed mainly of weakly cemented sand/sandstone and silt/siltstone. The average formation temperature in the hydrate zones is about 10° C., ranging from 6 to 14° C.
Methods have also previously been developed to estimate hydrate saturation from wireline induction resistivity logs. Logging-while-drilling (LWD) electromagnetic log results in hydrate-bearing formation in the Costa Rica continental margin have been previously reported. Dielectric estimates are a good proxy of in-situ hydrate saturation in, as a minimum, modeling hydrate dynamics. In the prior art, factors including the relatively low frequency (2 MHz) of the measurements did not allow assessment of the dielectric properties of in-situ gas hydrate formation. Dielectric measurements to quantify gas hydrate amounts in laboratory test media have been conducted, however, in-situ dielectric properties of natural gas hydrate have not been previously measured and reported.
Thus, there still remains a need to estimate in-situ hydrate saturation, in particular, determine in-situ dielectric properties of gas hydrate amounts.